Spinal neurons play a crucial role in generating voluntary limb movements, but since they have been studied primarily in anesthetized animals, little is known about their activity during normal movements. Moreover, they usually have been characterized with respect to reflex activation, so nothing is known about their possible role in movement preparation. We evaluated the extent to which spinal interneurons (INs) are engaged in preparation for voluntary movement by characterizing their responses in an instructed delay task. Two macaques were trained to control a cursor position by isometric flexion/extension torques about the wrist. Trials included a rest period, an instructed delay period, and an active torque hold period. Activity of isolated C6-T1 INs was recorded with one or two electrodes. The average firing rates of 123/394 INs showed significant delay period modulation (SDM) relative to rest (paired t-test, p<0.05). SDM was never accompanied by any detectable mus cle activity. Activity of 90% of INs with SDM was also modulated for active torques. Firing rates during the delay and torque periods were modulated in the opposite direction (i.e., increase vs. decrease) for 39% of INs with SDM. Firing rate decreased during the delay period for 61% of INs with SDM. INs exhibited SDM either during flexion trials only (n=39), extension trials only (n=54) or both (n=30). The latter group showed similar rate changes for pre-flexion and pre-extension delay periods (r =.89). These results indicate that preparation for movement involves extensive modulation of spinal interneurons that cannot be predicted from their responses during active movement. Thus, spinal circuitry, like cerebral cortex, is involved in the earliest stages of movement preparation, suggesting widely distributed, parallel processing of set-related activity. FUNDING NIH grants RR00166 and NS12542. Perlmutter, S., Maier, M., and Fetz, E.E. Activity and output linkages of spinal premotor interneurons during voluntary wrist movements in the monkey. J. Neurophysiol. 80 2475- 2494, 1998. Maier, M., Perlmutter, S. and Fetz, E.E. Response patterns and force relations of monkey spinal interneurons during active wrist movement. J. Neurophysiol. 80 2495-2530, 1998. Fetz, E.E., Smith, W., Prut, Y., Maier, M. and Perlmutter, S.I. Neuronal interactions in primate sensorimotor cortex and spinal cord during voluntary movements. Eur. J. Neurosci. 10 433, 1998 Prut, Y., Perlmutter, S.I. and Fetz, E.E. Activity of spinal interneurons during premovement delay period in primates. Soc. Neurosci. Abstr. 24 918, 1998. Kawashima, T., Knight, T.A. and Fetz, E.E. Interhemispheric synchronization of neuronal and field potential activity during bimanual movements of monkeys. Soc. Neurosci. Abstr. 24 403, 1998. Fetz, E.E. Dynamic activities in neuronal populations of primate sensorimotor cortex. Proc. Third Riken-Tamagawa International Dynamic Brain Forum., p. 8, 1998.